The present invention relates to a two-dimensional image detector capable of detecting images in radioactive rays (such as X-rays), visible light, infrared light, etc.
Conventionally, a known type of two-dimensional image detector for radioactive rays is a device comprising a two-dimensional arrangement of semiconductor sensors which detect X-rays and produce charge (electron-hole) pairs, each sensor being provided with an electrical switch, in which the electrical switches are sequentially turned ON by row, and the charge of each sensor in that row is read out.
The principle of such a two-dimensional image detector and specific structures therefor are discussed in, for example, D. L. Lee, et al, xe2x80x9cA New Digital Detector for Projection Radiographyxe2x80x9d (Physics of Medical Imaging, Proc. SPIE 2432, pp.237-249, 1995); L. S. Jeromin, et al, xe2x80x9cApplication of a-Si Active-Matrix Technology in a X-Ray Detector Panelxe2x80x9d (SID (Society for Information Display) International Symposium, Digest of Technical Papers, pp.91-94, 1997); and Japanese Unexamined Patent Publication No. 6-342098/1994 (Tokukaihei 6-342098, published on Dec. 13, 1994).
The following will explain the principle and structure of the foregoing conventional two-dimensional image detector for radioactive rays. FIG. 7 is a perspective view schematically showing the structure of the foregoing conventional two-dimensional image detector for radioactive rays. Further, FIG. 8 is a cross-sectional view schematically showing the structure of one pixel thereof.
As shown in FIGS. 7 and 8, the foregoing conventional two-dimensional image detector for radioactive rays includes an active matrix substrate made up of electrode lines (gate electrodes 52 and source electrodes 53) arranged in an XY matrix, TFTs (thin-film transistors) 54, charge storage capacitors (Cs) 55, etc., provided on a glass substrate 51. Further, over substantially the entire surface of the active matrix substrate 51 are provided a photoconductive film 56, a dielectric layer 57, and an upper electrode 58.
Each charge storage capacitor 55 is made up of a Cs electrode 59 and a pixel electrode 60 (which is connected to the drain electrode of the TFT 54), provided opposite each other but separated by an insulating layer 61.
The photoconductive film 56 is made of a semiconductor material which produces a charge when radioactive rays such as X-rays are projected thereon; the examples discussed in the foregoing documents use amorphous selenium (a-Se), which has high dark resistance and good photoconductive characteristics for X-rays. The photoconductive film 56 (a-Se) is formed by vacuum vapor deposition with a thickness of 300 xcexcm to 600 xcexcm.
For the foregoing active matrix substrate, it is possible to use active matrix substrates formed in the process of manufacturing liquid crystal display devices. For example, an active matrix substrate used in an active matrix liquid crystal display device (AMLCD) has a structure which includes TFTs made of amorphous silicon (a-Si) or polycrystalline silicon (p-Si), electrodes arranged in an XY matrix, and charge storage capacitors (Cs). Accordingly, it is easy to use such an AMLCD as an active matrix substrate for a two-dimensional image detector for radioactive rays, necessitating only minor design changes.
The following will explain the operating principle of the foregoing conventional two-dimensional image detector for radioactive rays.
When radioactive rays are projected onto the photoconductive film 56 of, e.g., a-Se, charge (electron-hole) pairs are produced therein. As shown in FIGS. 7 and 8, the photoconductive film 56 and the charge storage capacitor 55 are electrically connected in series, and thus if a voltage is applied between the upper electrode 58 and the Cs electrode 59 in advance, the electron and hole members of the charge pairs produced in the photoconductive film 56 move to the + and xe2x88x92 electrode sides, respectively. As a result, a charge accumulates in the charge storage capacitor 55. Between the photoconductive film 56 and the charge storage capacitor 55 is provided a carrier blocking layer 62 made of a thin insulating layer, which serves as a blocking photodiode for blocking a charge injection from one side to the other.
By putting the TFTs 54 in an open state by means of input signals from gate electrodes G1, G2, G3, . . . , Gn, the charges accumulated in the respective charge storage capacitors 55 due to the foregoing effect can be drawn out through source electrodes S1, S2, S3, . . . , Sn. Since the electrode lines (the gate electrodes 52 and the source electrodes 53), the TFTs 54, the charge storage capacitors 55, etc. are all provided in the form of an XY matrix, X-ray image information can be obtained two-dimensionally by sequential scanning of the signals inputted to the gate electrodes G1, G2, G3, . . . , Gn.
Incidentally, if the photoconductive film 56 used has photoconductivity not only for radioactive rays such as X-rays but also for visible light, infrared light, etc., the foregoing conventional two-dimensional image detector can also function as a two-dimensional image detector for visible light, infrared light, etc.
The foregoing conventional two-dimensional image detector for radioactive rays uses a-Se for the photoconductive film 56, but a-Se has the following drawbacks. First, since the transmission characteristics of photoelectric current by a-Se are of the dispersion type peculiar to amorphous materials, response is poor. Further, due to the insufficient sensitivity to X-rays (S/N ratio) of a-Se, information cannot be read out unless the charge storage capacitors 55 are sufficiently charged by long X-ray exposure.
Further, in the foregoing conventional two-dimensional image detector for radioactive rays, a dielectric layer 57 is provided between the photoconductive film 56 (a-Se) and the upper electrode 58 in order to prevent accumulation of a charge in the charge storage capacitors due to current leakage during X-ray projection, to reduce current leakage (dark current), and to protect from high voltage. However, since it is necessary to add a step (sequence) for eliminating a residual charge from the dielectric layer 58 after each frame, another drawback of the foregoing conventional two-dimensional image detector for radioactive rays is that it can only be used for pickup of still images.
In order to obtain image data corresponding to moving images, on the other hand, it is necessary to use, instead of a-Se, a photoconductive film 56 which is made of a crystalline (or polycrystalline) material, and which also has superior sensitivity to X-rays (S/N ratio). By improving the sensitivity of the photoconductive film 56, it is possible to charge the charge storing capacitor 55 with X-ray exposure of short duration, and since a high voltage need not be applied to the photoconductive film 56, the dielectric layer 57 itself is no longer necessary.
Known examples of this kind of photoconductive material with superior sensitivity to X-rays include CdTe and CdZnTe. Since photoelectric absorption of X-rays by a substance is generally proportional to the fifth power of its effective atomic number, if, for example, the effective atomic number of Se is 34 and that of CdTe is 50, then CdTe can be expected to have a sensitivity of approximately 6.9 times that of Se. However, if replacement of the a-Se in the foregoing conventional two-dimensional image detector with CdTe or CdZnTe is attempted, the following problems arise.
With conventional a-Se, a film can be formed by vacuum vapor deposition, and in this case, since the film formation temperature can be set at room temperature, it is easy to form a film on the foregoing active matrix substrate. With CdTe or CdZnTe, on the other hand, film formation by MBE (molecular beam epitaxy) or MOCVD (metal organic chemical vapor deposition) are known; in view of film formation on, in particular, substrates of large surface area, MOCVD is considered most suitable.
However, when forming a film of CdTe or CdZnTe by MOCVD, since the starting materials organic cadmium and organic tellurium have heat decomposition temperatures of approximately 300xc2x0 C. (for dimethyl cadmiumxe2x80x94DMCd) and approximately 400xc2x0 C. (for diethyl telluriumxe2x80x94DETe) or approximately 350xc2x0 C. (for diisopropyl telluriumxe2x80x94DiPTe), a high film formation temperature of around 400xc2x0 C. is needed.
The TFTs 54 formed on the foregoing active matrix substrate generally use semiconductor layers made of films of a-Si or p-Si, which, in order to improve semiconductor characteristics, are formed while adding hydrogen (H2) at a film formation temperature of 300xc2x0 C. to 350xc2x0 C. The heat resistance of a TFT element formed in this manner is approximately 300xc2x0 C., and exposure to higher temperatures causes the hydrogen to escape from the a-Si or p-Si film, thus impairing semiconductor characteristics.
Accordingly, in consideration of film formation temperature, it is in fact difficult to use MOCVD to form a film of CdTe or CdZnTe on the foregoing active matrix substrate.
It is an object of the present invention to provide a two-dimensional image detector which has good response, and which is capable of dealing with moving images.
In order to attain the foregoing object, a two-dimensional image detector according to the present invention comprises:
an active matrix substrate, provided with a pixel layer which includes pixel electrodes and switching elements arranged in matrix form;
a counter substrate which includes a first electrode section provided over substantially the entirety of the pixel layer, and a semiconductor layer having photoconductivity, provided opposite the pixel layer; and
conductive connection means which mutually connect the active matrix substrate and the counter substrate;
in which the active matrix substrate is provided with a second electrode section for inputting a signal to the first electrode section of the counter substrate; and
the first and second electrode sections are electrically connected to each other via electrode transition means.
With the foregoing structure, the first electrode section and the semiconductor layer are provided on the counter substrate, which is separate from the active matrix substrate, and the active matrix substrate and the counter substrate are electrically connected to each other by the conductive connection means.
Accordingly, a photoconductive film of a crystalline (or polycrystalline) material such as CdTe or CdZnTe, which have better sensitivity to X-rays (S/N) than a-Se, can be used as the semiconductor layer, without impairing the semiconductor characteristics of the switching elements formed on the active matrix substrate. As a result, it becomes possible to pick up moving images in real time.
Further, with the foregoing structure, the active matrix substrate is provided with a second electrode section for inputting a signal to the first electrode section provided on the counter substrate, and the first and second electrode sections are electrically connected to each other via electrode transition means. In other words, by inputting an electrical signal to the second electrode section, the electrical signal can be inputted into the first electrode section via the electrode transition means.
Incidentally, in typical two-dimensional image detectors, the periphery of the active matrix substrate is provided with bus line signal input terminals. Since an external driving LSI (large scale integrated circuit), data readout LSI, etc. must be connected to these terminals, the active matrix substrate is generally larger in size than the counter substrate. Accordingly, with the foregoing structure, the second electrode section provided on the active matrix substrate can be easily connected to external signal input means. Consequently, an electrical signal can be easily inputted to the first electrode section via the electrode transition means.